A known semiconductor-based optical phase-modulating device comprises a plurality of semiconductor layers defining an optical waveguide section, and a pair of phase-modulating electrodes on opposite sides of the waveguide section. A time-varying electric signal is applied to the phase-modulating electrodes to apply an electric field across the waveguide section. The electric field modulates refractive indices of the semiconductor layers, thereby modulating the effective optical path length through the waveguide section and the resulting phase of the optical signal at an output end of the waveguide section.
Such known optical phase-modulating devices can be used in the construction of integrated optic devices. For example, an integrated optic Mach-Zehnder interferometer can be constructed from two such optical phase-modulating devices connected between a waveguide splitter and a waveguide combiner. An optical signal is split into two components by the waveguide splitter, each component is passed through a respective phase-modulating device, and the phase-modulated signals are recombined by the waveguide combiner to produce a recombined signal. By applying different electric signals to the phase-modulating electrodes of the two phase-modulating devices, the relative phase of the optical signal components is modulated resulting in intensity modulation of the recombined signal.
The semiconductor layers which define the optical waveguide section have a finite resistance. Consequently, electric signals applied to the phase-modulating electrodes spread along the semiconductor layers beyond ends of the phase-modulating electrodes. The distributed resistance and capacitance of the semiconductor layers act as a distributed RC network, so that the phase-modulation provided by the phase-modulation device has an undesirable frequency dependence. In Mach-Zehnder interferometers constructed from such phase-modulating devices, the frequency dependence of phase-modulation results in frequency dependence of the modulation depth of the recombined signal.